Add ability to append RenderFragment into a RenderTreeBuilder

src/Microsoft.AspNetCore.Blazor/RenderTree/RenderTreeBuilder.cs

@@ -65,6 +65,25 @@ namespace Microsoft.AspNetCore.Blazor.RenderTree
         public void AddContent(int sequence, string textContent)
             => Append(RenderTreeFrame.Text(sequence, textContent ?? string.Empty));
 
+        /// <summary>
+        /// Appends frames representing an arbitrary fragment of content.
+        /// </summary>
+        /// <param name="sequence">An integer that represents the position of the instruction in the source code.</param>
+        /// <param name="fragment">Content to append.</param>
+        public void AddContent(int sequence, RenderFragment fragment)
+        {
+            if (fragment != null)
+            {
+                // We surround the fragment with a region delimiter to indicate that the
+                // sequence numbers inside the fragment are unrelated to the sequence numbers
+                // outside it. If we didn't do this, the diffing logic might produce inefficient
+                // diffs depending on how the sequence numbers compared.
+                OpenRegion(sequence);
+                fragment(this);
+                CloseRegion();
+            }
+        }
+
         /// <summary>
         /// Appends a frame representing text content.
         /// </summary>

test/Microsoft.AspNetCore.Blazor.Test/RenderTreeBuilderTest.cs

@@ -342,6 +342,33 @@ namespace Microsoft.AspNetCore.Blazor.Test
                 frame => AssertFrame.Text(frame, "Goodbye", 6));
         }
 
+        [Fact]
+        public void CanAddFragments()
+        {
+            // Arrange
+            var builder = new RenderTreeBuilder(new TestRenderer());
+            RenderFragment fragment = fragmentBuilder =>
+            {
+                fragmentBuilder.AddContent(0, "Hello from the fragment");
+                fragmentBuilder.OpenElement(1, "Fragment element");
+                fragmentBuilder.AddContent(2, "Some text");
+                fragmentBuilder.CloseElement();
+            };
+
+            // Act
+            builder.OpenElement(10, "parent");
+            builder.AddContent(11, fragment);
+            builder.CloseElement();
+
+            // Assert
+            Assert.Collection(builder.GetFrames(),
+                frame => AssertFrame.Element(frame, "parent", 5, 10),
+                frame => AssertFrame.Region(frame, 4, 11),
+                frame => AssertFrame.Text(frame, "Hello from the fragment", 0),
+                frame => AssertFrame.Element(frame, "Fragment element", 2, 1),
+                frame => AssertFrame.Text(frame, "Some text", 2));
+        }
+
         [Fact]
         public void CanClear()
         {

test/testapps/BasicTestApp/RenderBlockComponent.cs

@@ -48,9 +48,7 @@ namespace BasicTestApp
 
             if (_showRegion)
             {
-                builder.OpenRegion(3);
-                _exampleContent(builder);
-                builder.CloseRegion();
+                builder.AddContent(3, _exampleContent);
             }
 
             builder.OpenElement(4, "button");